In the manufacture of integrated circuits, the planarization of semiconductor wafers is becoming increasingly important as the number of layers used to form integrated circuits increases. For instance, metallization layers formed to provide interconnects between various devices may result in nonuniform surfaces. The surface nonuniformiities may interfere with the optical resolution of subsequent lithographic steps, leading to difficulty with printing high resolution patterns. The surface nonuniformities may also interfere with step coverage of subsequently deposited metal layers and possibly cause open or shorted circuits.
Various techniques have been developed to planarize the top surface of a semiconductor wafer. One such approach involves polishing the wafer using a polishing slurry that includes abrasive particles mixed in a suspension agent. With this approach, the wafer is mounted in a wafer holder, a polishing pad has its polishing surface coated with the slurry, the pad and the wafer are rotated such that the wafer provides a planetary motion with respect to the pad, and the polishing surface is pressed against an exposed surface of the wafer. The polishing erodes the wafer surface, and the process continues until the wafer is largely flattened. Typically, the slurry is introduced near the center of the pad, forms a ring around the wafer and goes under the wafer as necessary. It is generally desirable to maintain an adequate amount of slurry between the wafer and the pad while dispensing as little slurry as possible to lower costs.
In chemical-mechanical polishing, the slurry particles abrade the wafer surface while a chemical reaction occurs at the wafer surface. For instance, in chemical-mechanical polishing of silicon dioxide, the slurry particles generate high pressure areas that cause the silicon dioxide to react with water. In chemical-mechanical polishing of other materials, such as tungsten, the slurry employs a wet chemical etchant to assist in removing wafer material. The wet chemical etchant is often more selective to the exposed wafer material than to underlying wafer materials.
The polishing pad can be a felt fiber fabric impregnated with polyurethane, with the amount of impregnation determining whether the pad is a "hard pad" or a "soft pad." A hard pad tends to focus the polishing pressure on protruding regions of the wafer surface in order to rapidly planarize the wafer surface. A soft pad tends to create a more even polish over the entire wafer surface, a finer surface finish, and less mechanical damage to the wafer.
A significant goal relating to chemical-mechanical polishing techniques is the maintenance of substantially uniform planarity over the entire surface of a given wafer. Due to problems which will be described in the present application, uniformity is particularly difficult to achieve near the edge of a given wafer.